This invention relates to compositions of absorbable polymers containing a bioabsorbable filler. More specifically, it relates to absorbable polymer compositions containing a reinforcing filler which enhances the stiffness of the polymer composition, yet decomposes into components which are biocompatible with bodily tissue.
The need to replace surgical and medical devices made of metallic components continues to grow as surgical procedures become more intricate and complex. The driving force for the replacement of such metallic devices is the need for devices composed of materials which are capable of being absorbed by the body. Bioabsorbable materials obviously represent a significant advantage over metallic materials, in that bioabsorbable materials do not need to be removed after their surgical function has been accomplished. In contrast, metallic devices remain in the body and often require removal when the surgical repair is completed to prevent possible adverse reactions occurring due to the prolonged contact of the metallic device and the surrounding bodily tissue or due to the byproducts of the corrosion of the metal.
As a result of the burgeoning need for bioabsorbable materials in surgery and for other medical applications, a body of art has been developed which utilizes bioabsorbable polymers as the structural component of these devices. In this manner, once the device has performed its function, the bioabsorbable polymer from which it is composed readily breaks down into nontoxic segments which can be metabolized or passed through bodily tissue. For example, U.S. Pat. No. 4,052,988 describes preparing absorbable surgical devices from polymers of 1,4-dioxanone and 1,4-dioxepan-2-one. The devices which can be prepared from these absorbable polymers include sutures, tubular implants, surgical meshes, staples, and cylindrical pins, rods or screws. The properties of the polymers from which the devices are made can be changed by copolymerizing 1,4-dioxanone or 1,4-dioxepan-2-one with other lactone monomers, such as lactide or glycolide, or by forming mixtures of the homopolymers with other absorbable polymers.
Other examples exist of the use of bioabsorbable polymers as the main component for surgical devices. U.S. Pat. No. 4,741,337 describes surgical fasteners, particularly staples, composed of a polymeric blend derived from homopolymers and copolymers of lactide and glycolide. The blending of the polymers is optimized to yield fasteners which can retain their strength in vivo for prolonged time periods, yet become impalpable shortly thereafter.
Another example of surgical devices made from absorbable polymers can be found in U.S. Pat. No. 4,646,741. This patent describes surgical fasteners made from polymeric blends. The blends contain a copolymer of lactide and glycolide and a homopolymer of 1,4-dioxanone. Once again, the proportion of polymers in the blend is carefully controlled to achieve the optimum properties for the fastener.
While absorbable, polymeric surgical and medical devices represent an advantage over metallic devices because the polymeric devices do not need to be removed from the body, such polymeric devices often have a major drawback which has limited their applications. Specifically, absorbable polymers typically lack the strength and stiffness of metallic components. Strength is an important asset for devices designed to penetrate bodily tissue or to withstand heavy loads. For these applications, the absorbable polymers must be stiff enough to withstand the penetration forces or the load placed on them. With respect to this important attribute of stiffness, absorbable polymers are usually incapable of matching the performance characteristics of metals and metal alloys used for surgical devices.
Accordingly, attempts have been made to increase the stiffness of bioabsorbable polymers from which surgical and medical devices are made. U.S. Pat. No. 4,473,670 describes preparing absorbable polymers containing finely divided sodium chloride or potassium chloride for surgical devices such as ligating clips and staples. The salt filler enhances certain properties of the polymer, most notably its stiffness. The absorbable polymers which can be used include homopolymers and copolymers of lactide, glycolide, and 1,4-dioxanone. In a similar manner, U.S. Pat. No. 4,612,923 discloses another example of using a filler to increase the stiffness of an absorbable polymer. In this case, an absorbable glass is used as the filler.
Unfortunately, the use of bioabsorbable glasses or inorganic salts as fillers for bioabsorbable polymers has certain disadvantages. First, since the absorbable polymer matrix is organic material, and the fillers described in these patents are inorganic compounds, the adhesion between the absorbable polymer matrix and the filler may be less than desirable for adequate performance. That is, a lack of adhesion between the filler and the polymer matrix will tend to reduce the synergistic effects of their combination, and significant improvements in stiffness may not be realized. Second, the use of an absorbable glass filler may cause the calcification of soft tissue when the device from which the glass filled polymer degrades inside the body.
In view of the deficiencies of the prior art, what is needed is a bioabsorbable filler for absorbable polymers in which the filler can readily break down into biocompatible segments. In addition, and most importantly, what is also needed is an organic compound that is used as a reinforcing filler and is compatible with the absorbable polymer matrix so that good adhesion and blending can be established for the optimum improvement in the properties, especially the stiffness of the polymer.